[credentialless]: Fix WPT reliability.

It sounds like some server of the official web-test-platform runner can
drop some requests when too many are sent in parallel.

This patch ensures both send/receive function retry as long as there is
an error. We also limit the number of concurrent access to the fetch API
to one per document.

Bug: 1196073
Change-Id: I2c2ef6c2b7c6d7bbeeea2709f1c6bacbcb081419
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2807579
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Pâris Meuleman <pmeuleman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#869558}